The present invention relates to a passive acoustics process to monitor the feed injection lines of a catalytic cracker.
The catalytic cracking of heavy oils to produce gasoline is widely accepted as the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products. Cracking can be defined as the breakdown of higher molecularweight hydrocarbons to lighter components by the application of heat. Cracking in the presence of a suitable catalyst produces an improvement in yield and quality over simple thermal cracking.
Catalytic cracking is a continuous fluid bed process where, at catalyst mass rates as high as 40 tons a minute, fresh catalyst is brought into contact with injected feed and subsequently separated as deactivated ("spent") catalyst from the gas stream of cracked hydrocarbons. This separation occurs in a fluidized bed reactor and stripper. The spent catalyst in then reactivated by oxidation of coke deposits in a fluid bed regenerator and returned to the injection zone as fresh catalyst.
The transfer of catalyst particles between the fluidized bed of the regenerator and other parts of the system is through transfer lines where the particles are transported in high velocity gas streams. The most important of these transfer lines from the viewpoint of feed conversion and product yield is the feed riser. In the feed riser initial contact is made between the fresh catalyst and an injected stream of oil and steam and a complex physical/chemical process takes place which produces large quantities of gas due to: (1) oil vaporization on the hot catalyst and (2) catalytic and thermal cracking of light hydrocarbons from the more complex hydrocarbons in the injected oil.
There are three sections of the feed riser ideally performing three critical functions:
An Initial Injection Zone where oil is introduced, contacts the circulating fresh catalyst and is vaporized. The design and performance of feed nozzles that inject the two phase mixture of oil and steam into the catalyst stream is critical for the performance of this section as is the density and uniformity of the catalyst particles.
An Intermediate Reaction Zone where the major part of the catalytic cracking reaction is carried out and whose function is to maintain good contact between feed and catalyst while avoiding excessive back mixing. A critical design parameter is the average catalyst velocity.
A Final Transport Zone which has the function of cracking the remaining convertible material without re-cracking valuable products (such as gasoline) that have been produced in the intermediate section.
Fluid bed catalytic cracking units (cat-crackers or FCCU's) are designed to run continuously over periods of many years between planned shut-downs for maintenance, design changes and repairs. Due to the large throughput of cat-crackers, even modest improvements in performance, if maintained through the run length, are converted into significant financial incentives.
The complex physical and chemical processes that take place in the various sections of the feed riser, present a significant obstacle in optimizing the yield and product distribution of the cat-cracking process against changes in the quantity and quality of the oil feed as well as changes in catalyst physical and chemical properties. In addition, changes in the performance of critical components such as feed nozzles and steam injection nozzles are difficult to detect. It is thus possible for the cat-cracker to operate at significant departures from "design" or "standard" conditions for extended periods of time. Therefore, there is a significant need for a real-time monitoring system that would alert the operator to departures of the process from optimum operating conditions and assist in optimizing conversion and yield of the incoming feed against changes in the operation of the unit.
Sampling of the cat-cracker output and on-line chemical analysis of the sample might appear to be the most direct route toward meeting this need. Unfortunately current limitations on analytical sensitivity and accuracy rule out sampling as an on-line monitor of cat-cracker conversion and yield vector.
Moreover, on-line extraction of a representative product from a working cat-cracker, even assuming significant improvements in analytic technology, does not meet the above need. An on-line extraction system would be intrusive and require penetration of the wall of the feed riser. The extraction probes would be subject to the harsh physical and chemical environment of the feed riser and could easily clog. Furthermore there is no assurance that the output of such a local probe would be representative of the "global" performance of the unit.
Local probes that give a global measure of the performance of a cat-cracker have been introduced. U.S. Pat. No. 4,650,566 and U.S. Pat. No. 4,808,383 describe the use of temperature probes to obtain uniform temperature distributions within the feed riser which correlate with improved unit performance. Such probes are not directly coupled to the catalytic cracking reaction, but rather to flow uniformities.
There thus remains a continuing need for a robust, non intrusive, on-line monitor of the cat-cracking reaction in the feed riser. The present invention satisfies this need.